'Black box' is generally orange and far from cockpit

Usually painted bright orange to help easy retrieval, a black box is a combination of two devices mounted at the rear of an aircraft not the cockpit, since that is the area normally least affected during air crashes, experts explain.

It is called black box because of the tragic circumstances in which the system is generally retrieved and has nothing to do with the colour of its outer casing.

In aviation parlance, it is actually a loose term used for two crucial pieces of equipment -- the digital cockpit voice recorder and the flight data recorder -- that give crucial inputs to investigators on the causes of air mishaps.

One device digitally records all conversations inside the cockpit and those with the air traffic controllers, among its other uses, and gives vital clues to the cause of any air disaster.

The other has the history of the aircraft's flight details, such as acceleration, engine thrust, airspeed, altitude, rudder position, which are also vital for crash probes.

A sturdy system the size of a shoebox, it has a reflective tape on its exterior and can withstand extreme temperatures. It has a solid steel encasing and heat-resistant material to withstand heavy impact and razing fire, the experts said.

The box also has an underwater locating device, which gets activated immediately upon contact with water and helps in its retrieval should an aircraft sink in a water body.

"The recorded data can be analysed for the purpose of checking deviations in flight parameters beyond acceptable limits which are critical to flight safety," says one of the manuals compiled by the Directorate General of Civil Aviation (DGCA).

Experts at the DGCA will decode the black box from the Air India aircraft in the national capital. If for some reason, the data retrieval becomes difficult, they will take the help of aircraft manufacturer Boeing.

The US-based manufacturer has already dispatched a team of experts to assist in the technical investigations.